After being held out of the starting lineup four consecutive games for mental and physical rest, Gregory Polanco returned last Sunday in Milwaukee.

Brewers starter Mike Fiers struck out Polanco in the first inning then challenged him with a 91 mph, middle-of-the-plate fastball in the third. It was a perfect pitch to hit. Polanco was late, his swing traveled on a long and looping path, and he whiffed for another strikeout. Polanco dropped his bat in disgust and tossed his batting helmet to the dirt, rare demonstrative actions from the rookie. He struck out in all three of his plate appearances. The following day, he was sent back to Triple-A Indianapolis, from where he was summoned in June, billed as baseball's next big thing. Polanco had one hit in his last 30 at-bats.

Polanco's slump follows the performance track of many young players. Fellow highly rated prospects Xander Bogaerts (Boston Red Sox), Oscar Taveras (St. Louis Cardinals) and Jackie Bradley Jr. (Red Sox) also struggled this season. They perhaps are dealing with a phenomenon known as the rookie wall. They are reminders that not every rookie performs like Yasiel Puig.

“Is it real? I do think there is some merit to it,” said Pirates manager Clint Hurdle of the rookie wall. “I do think there can be fatigue. The mental part of it can be taxing. I do think the information that is available, the way you can identify weaknesses, is part of it. I think more often than not young players do run into some challenges.”

Is the rookie wall real? A Tribune-Review analysis of hundreds of thousands of plate appearances by rookie batters during the past 30 years suggests the performance barrier exists, and the wall is appearing earlier in careers and becoming more formidable to scale.

The speed of the counterpunch

There are no secrets.

Data has revolutionized scouting. Data-based, color-coded charts illustrate strengths and weaknesses of a batter against certain pitches in certain areas of the strike zone.

Polanco has struggled with elevated fastballs. With the advent of pitch-tracking system PITCHf/x in 2007, the sophistication level and volume of scouting data increased dramatically. The book on rookies now is written quickly.

The Tribune-Review studied two groups of rookie batters: those who debuted in the PITCHf/x era, from 2007 to this season, and rookies who played from 1980 to 1994, the last generation to reach the majors before the proliferation of video scouting and information sharing on the Internet. Their performances were studied at 30-game segments to identify peaks and lulls. (The study included batters who played in at least 150 games).

The Tribune-Review found the PITCHf/x generation has hit the rookie wall sooner than a previous generation of batters. The 2007-14 batters' combined on-base plus slugging was .727 in their second 30-game segment and the OPS dipped to .718 in the next sample. By 60 games into a player's career, the league generally finds flaws and punches back. Polanco was sent down after his 64th game played.

Hurdle said Polanco is a prime example of how quickly the league adjusts with today's data.

Polanco hit .288 in June but just .214 in July.

“You look at Polanco, two weeks in the big leagues, he was as hot as you could be,” Hurdle said. “(Then) they had plans coming out with all the different video. They saw the hot spots. They have counterpunched. ... Somewhere around the 50, 75 at-bat mark, you can pretty much get a real good read (on a rookie batter).”

The group of rookie batters from 1980-1994 hit a performance wall roughly a month later, from their 90th to 120th career games.

“(Scouting) took a lot longer because it was word of mouth,” Hurdle said of his playing days. “Ballplayers are capable of gossiping, believe it or not. That was the way information was shared back in the day. It would have been a phone call. It would be before a game, ‘Hey, you are going to play so-and-so, here's what we got because we don't like them or they are in our division.' That's the way it was. It would take longer.”

Moreover, in the PITCHf/x era, the rookie wall has become more of a plateau of struggle.

Rather than generally improving as one would expect with experience, the young batters' performances declined as they gained at-bats in their first 150 games. Their collective OPS fell four consecutive periods until reaching a low of .712 as they reached 150 games played.

Said Pirates third baseman Josh Harrison: “In the minor leagues, if you're going hot, the team you're playing next is not going to know until you play them. In the big leagues, if you're going hot, all other 29 teams know.”

The 1980-94 sample group adjusted and significantly improved after hitting the wall. Their OPS climbed from .661 during their 90th through 120th games played to .748 during their 120th to 150th games played.

Dragging and lagging

Not only does the volume of scouting data change in graduating from the minors to the major leagues, but so does the volume of work with more games and a longer spring training. Harrison said it's not just the games that are taxing.

“The whole up-and-down thing is physically tiring as well as mentally. You are not just physically going from Indy to Pittsburgh,” Harrison said. “It's mental, (thinking) about my car, my wife, my kids. You have other things you have to put in place. … You are constantly living out of suitcase.

“I wouldn't trade it for the world. The big leagues is where we want to be. … (but) the big leagues is a grind.”

Hurdle believes Polanco has tired in the second half and his bat has slowed. Pirates hitting coach Jeff Branson said Polanco has lost his timing.

“When your timing is off, your body is going to get involved and it is going to create a longer swing,” Branson said. “He (went) down to get timing.”

Since April 2013, Polanco has tallied 1,356 plate appearances across all levels.

Pirates general manager Neal Huntington said he had hoped to shorten Polanco's winter ball stint, but Polanco played too well in the Dominican, earning MVP honors.

“He wanted to keep playing,” Huntington said. “We weren't really in position to shut him down.”

Ready or not

The Pirates hoped they never would have to send Polanco back to Indianapolis. Huntington said they wanted Polanco to “thrive,” not “survive,” when he reached the major league level. Teams never want young players to lose their confidence.

There also is a mental aspect in overcoming the rookie wall, with players perhaps facing the first significant failures of their playing careers. Andrew McCutchen faced his first challenges in the minor leagues. He slumped in Double-A in 2008 and recorded more than 1,300 plate appearances at Triple-A. Polanco never experienced failure at the upper levels of the minors. He breezed through the minors having had just 600 at-bats above A-ball.

“The game is mental. You have to be strong in that area. You have to know, No. 1, you are good enough,” McCutchen said. “You just need to maintain your strong points. It's going to make you more consistent.”

Polanco is expected to be recalled shortly. Can he get over the wall in September? On the surface, Polanco's performance is concerning.

In an age when rookies are more productive at the very beginning of the careers, Polanco produced a .657 OPS through his first 64 games, well below an average rookie debut of those studied.

The 2007-14 rookies produced a .724 OPS (league average OPS of .731 during period) in their first 60 games compared to a .670 OPS of the 1980-94 group (league average OPS of .714). And according to recent history, PITCHf/x-era rookie batters decline before they get better.

Still, there is a reason the Pirates wanted to sign Polanco to a seven-year deal before his first major league plate appearance. He's a rare talent. But what Polanco has proven to date is that he's not an exception to meeting the wall.

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